1. Field of the Invention
The present invention relates to an ink jet printing apparatus configured to perform printing by ejecting ink, and also relates to an ink jet printing method.
2. Description of the Related Art
An ink jet printing method configured to eject ink from an ink jet printing head to a printing medium and thereby to print an image on the printing medium has heretofore been known. This printing method has advantages including high-speed printing, high-density printing, and ease of color-image printing.
A typical ink jet printing head applies a method of ejecting ink from an ink ejection port by utilizing heat generation of an electrothermal converter (a heater). The printing head of this type is configured to apply a voltage to the heater to generate heat, to make the ink inside ink passage foam by use of that heat energy, and to eject the ink out of the ink ejection port by use of that foaming energy.
The amount of ink ejection of an ink jet printing apparatus using the above-described printing head may fluctuate as the viscosity of the liquid ink or its volume upon foaming changes depending on the temperature inside the printing apparatus and on that of the printing head. For example, a low-temperature printing head makes the amount of ink ejection reduced. As a result, density of a printed image may become lower than intended. On the other hand, when a high-temperature printing head makes the amount of ink ejection increased. As a result, density of a printed image may become higher than intended. In addition, when printing an image by use of plural printing heads, density of such a printed image may fluctuate from part to part depending on a difference in the temperature among the printing heads.
Moreover, the amount of ink ejection is also influenced by uneven heat conductivity among the heaters (hereinafter referred to as a “heater rank”) attributable to unevenness in the resistance value and the like. In the course of manufacturing the printing heads, resistance values of electrothermal conversion elements constituting the heaters may differ to some extent, and the difference in the resistance value causes a difference in the energy inputted to the heaters required for ejecting a predetermined amount of ink (ejection threshold energy). Accordingly, the size of the ejected ink droplet may differ among ejection ports even when the same drive voltage is applied to the plural heaters to which the ejection ports correspond.
The double-pulse-drive control is a known technique for stabilizing the amount of ink ejection.
In the double-pulse drive control, a predetermined drive voltage pulse is applied to a heater in the form of two pulses. The first pulse is a pre-heat pulse for allowing the heater to generate the heat to the extent not causing ink ejection so as to adjust the ink temperature in the ink passage. The second pulse is a main heat pulse for allowing the heater to generate enough heat to eject the ink. It is possible to stabilize the amount of ink ejection by adjusting the pulse width of the pre-heat pulse, the pulse width of the main heat pulse, and the interval of these pulses (interval time). For example, the pulse width of the pre-heat pulse is adjusted to be longer in the case where the amount of ink ejection is less than intended as the temperature of the printing head is too low. On the other hand, the pulse width of the pre-heat pulse is adjusted to be shorter in the case where the amount of ink ejection is more than intended as the temperature of the printing head is too high.
Alternatively, Japanese Patent Application Laid-open No. 2001-180015 discloses a method of controlling the amount of ink ejection by changing simultaneously the drive voltage and the drive pulse length of the printing head in response to print data.
However, when a continuous printing operation brings about a rise in the temperature of the printing head, which keeps rising even higher, it may be hardly possible to suppress the increase in the amount of ink ejection only by reducing the width of the pre-heat pulse. After the pulse width of the pre-heat pulse is reduced to zero, the printing head is subject to single-pulse-drive control. Under the single-pulse-drive control, it is difficult to reduce the amount of ink ejection thereafter.
Furthermore, no technique of drive control which responds to a temperature rise on the printing head is disclosed in Japanese Patent Application Laid-open No. 2001-180015. No technique of stabilizing the fluctuating amount of ink ejection, which is attributable to the difference in the heater ranks of the printing heads, and the like, is disclosed, either.